Direct view and projection display systems conventionally employ kinescopes as display devices. In normal operation of a kinescope an electron beam is deflected by scanning circuitry to produce a relatively large area raster on the face plate of the kinescope and video modulation of the beam produces a visible picture by activating phosphors deposited on the face plate. The beam energy in normal operation is therefore distributed over the whole area of the kinescope face plate. If scan loss should occur, this energy may be concentrated in a relatively small area and this high concentration of energy may produce permanent damage to the phosphor. This condition is commonly referred to as a kinescope "spot burn". Scan loss may occur during initial turn-on of a receiver or monitor under so-called "hot start"conditions. It may also occur during turn-off and it may also occur during normal operation of the kinescope due, for example, to a component failure.
It is known, generally, protect against scan loss by detecting or "predicting" the scan loss occurrence and, in response to the detected or predicted occurrence, to apply a negative grid bias to the kinescope of a value sufficient to blank or cut-off the electron beam. Circuits which control the grid bias and provide beam cut-off are sometimes referred to as so-called "grid kicker" circuits. Such circuits generally employ a capacitor which is charged to a relatively high voltage during normal kinescope operation. During scan loss conditions, the positive plate of the capacitor is clamped to ground to generate a high negative voltage that is applied to the kinescope grid for blanking the beam.
An example of spot burn protection by the grid cut-off technique is described by Valdes U.S. Pat. No. 4,340,910 entitled CRT SPOT SUPPRESSION CIRCUIT which issued Jul. 20, 1982. In this circuit, a scan indicating signal is applied via a parallel resistor and capacitor to the anode of a PN diode and to the control grid of a kinescope. The cathode of the diode is grounded. In operation, the scan indicating signal charges the capacitor and a portion of the signal flows through the parallel connected resistor to develop a positive grid bias voltage across the PN diode of about 0.6 volts. Upon scan loss, the reduction in scan indicating voltage is coupled via the charged capacitor to the diode thereby reverse biasing the diode and driving the kinescope grid negative to achieve grid cut-off thereby preventing spot burn of the kinescope.
Another example of a "grid kicker" circuit is described by Haferl in U.S. Pat. No. 4,488,181 entitled ELECTRON BEAM SUPPRESSION CIRCUIT FOR A TELEVISION RECEIVER which issued Dec. 11, 1984. In this example a "grid kicker " circuit is activated in a remotely controlled receiver upon switching between normal and standby receiver operating modes to thereby blank the kinescope prior to disablement of the receiver scanning circuits. In a specific embodiment of the Haferl apparatus the grid bias control circuit comprises a capacitor having a first plate connected to a charging source and having a second plate connected to the kinescope grid and coupled to ground via a PN diode. In normal operation the capacitor is charged by the charging source and a resistor in parallel with the capacitor applies forward bias to the diode thereby establishing a positive grid bias voltage for the kinescope of about 0.6 volts. Grid blocking is provided by a clamp transistor which clamps the first plate of the capacitor to ground in response to the turn-off (i.e., stand-by operating mode) command produced by the remote control unit. Accordingly, the diode is reverse biased and the kinescope grid is driven to a negative potential.
In the foregoing examples of grid bias control circuits the grid was biased at relatively modest positive potential provided by a forward biased PN diode during normal operation. This is the usual bias condition for kinescopes having cathodes driven by amplifiers capable of driving the cathode to nearly ground potential. Not all amplifiers have this capability. For example, certain cascode cathode driver amplifiers have an inherent limitation in the minimum output voltage they can produce. In order to achieve maximum brightness during normal kinescope operation it is necessary to provide a grid bias of several volts (e.g., 25 volts) and grid bias control circuits meeting this need have been developed.
A first example of a grid bias control circuit providing a predetermined positive grid voltage during normal operation is described by Gurley and Wignot in allowed U.S. patent application Ser. No. 515,512 entitled VIDEO DISPLAY APPARATUS WITH KINESCOPE SPOT BURN PROTECTION CIRCUIT which was filed Apr. 30, 1990, and is incorporated herein by reference. The Gurley and Wignot circuit is similar to those previously discussed but includes a potential divider network coupled between a source of high voltage and ground and having an output coupled to the kinescope grid. The network elements are selected to bias the grid at about 25 volts during normal operation. The network also includes a Zener diode having a break-down voltage of about 27 volts which clamps the grid voltage to that value at the end of a scan loss interval to prevent the charging current supplied to the capacitor from producing excessive positive grid bias. Advantageously, the Zener diode is biased off during normal operation to thereby prevent production of radio frequency interference (RFI) which otherwise may occur due to flow of current through the Zener diode.
Another example of a grid bias control circuit featuring potential divider control of the positive grid bias voltage and Zener diode limiting is described by Normal et al. in allowed U.S. patent application Ser. No. 516,385 entitled PROJECTION TV DEFLECTION LOSS PROTECTION CIRCUIT which was filed Apr. 30, 1990 and is incorporated herein by reference. In an embodiment of a projection television receiver they disclose the scan loss indication signal is obtained by a detector responsive to horizontal scanning pulses for controlling a high voltage PNP switching transistor. During normal operation, when the pulses are present, the transistor is turned on thereby supplying charging current to a "grid kicker" capacitor and supplying operating voltage to a potential divider network that biases the grid to a positive value of about 25 volts. When the horizontal sweep pulses are absent, the switching transistor turns off and a "pull down" resistor grounds the positive plate of the capacitor thereby driving the grid negative. In the specific example shown of the grid bias control circuit one resistor of the potential divider is connected in parallel with the capacitor. This connection determines the RC time constant for the negative output pulse produced by the circuit. For the specific circuit values given (e.g., 2.7 meg-Ohms and 4.7 micro-Farads), the time constant is about 12.7 seconds.